Racemic (±)-10,11-dihydro-10-hydroxy-5H-dibenz/b,f/azepine-5-carboxamide (III) has been shown to possess anti-convulsant activity (Schutz, H. et al., Xenobiotica, 16, 769-778 (1986)), and is the principal metabolite of the established anti-epileptic drug oxcarbazepine (IV). This racemate (III) serves as a useful intermediate for the preparation of optically pure (S)-(−)-10-acetoxy-10,11-dihydro-5H-dibenz/b,f/azepine-5-carboxamide (V) and (R)-(+)-10-acetoxy-10,11-dihydro-5H-dibenz/b,f/azepine-5-carboxamide (formula VI), two more recently disclosed, single-enantiomer putative anti-epileptic drugs demonstrating improved biological properties (Benes, J. et al., J. Med. Chem., 42, 2582-2587 (1999)). The (S)-(−)-enantiomer (V) in particular has been shown to display a very favourable anti-convulsant profile.

A key step in the synthesis of either of the optically pure individual acetate esters (V) or (VI) involves the resolution of racemic (±)-10,11-dihydro-10-hydroxy-5H-dibenz/b,f/azepine-5-carboxamide (III) into its individual, optically pure stereoisomers, (S)-(+)-10,11-dihydro-10-hydroxy-5H-dibenz/b,f/azepine-5-carboxamide (I) and (R)-(−)-10,11-dihydro-10-hydroxy-5H-dibenz/b,f/azepine-5-carboxamide (II), which are the principal intermediates for synthesis of the enantiomerically pure acetates (V) and (VI). An improved method for this resolution was recently disclosed involving the efficient separation of diastereoisomeric tartrate half-esters of racemic (±)-10,11-dihydro-10-hydroxy-5H-dibenz/b,f/azepine-5-carboxamide (III) (Learmonth, D., PCT/GB02/02176).
Racemic (±)-10,11-dihydro-10-hydroxy-5H-dibenz/b,f/azepine-5-carboxamide (III) can be easily prepared by reduction of the ketone group of oxcarbazepine (IV), by the use of, for example, metal hydrides in alcoholic medium. However, oxcarbazepine (IV) is an expensive substance, and despite the very efficient resolution procedure (around 98% yield based on a single diastereoisomer), development of say only the (S)-(−)-acetate (V) would mean the loss of approximately 50% of costly material. It would thus be highly desirable to have a method of recycling this unwanted, but expensive (R)-(−)-10,11-dihydro-10-hydroxy-5H-dibenz/b,f/azepine-5-carboxamide (II) which can be recovered from the resolution mixture. However recycling of this material is very complicated due to the propensity for elimination of water across the C10-C11 junction even under very mild conditions, which provides an olefinic product of negligible economic interest. Notwithstanding, recycling could be envisaged to involve inversion of the chiral centre at C-10 by a Mitsunobu reaction protocol with concomitant esterification (Mitsunobu, O., Synthesis, 1-29, (1981)), whereby the recovered but unwanted optically enriched (R)-(−)-10,11-dihydro-10-hydroxy-5H-dibenz/b,f/azepine-5-carboxamide (II) is converted directly to the (S)-(−)-acetate (V) or to analogous chirally inverted ester derivatives of potential biological interest. The Mitsunobu procedure should preferably involve the use of readily available solvents and reagents, and be operationally simple whilst affording good yields of chirally-inverted, esterified products. Additionally, it would be highly desirable for large-scale manufacturing purposes to develop the Mitsunobu inversion reaction so as to obtain the desired inverted products in high purity and yield through a significantly simplified purification process without resort to inconvenient and tedious purification by column chromatography over silica gel which is usually required to remove unwanted reagents and by-products associated with the Mitsunobu reaction, such as, for example, triphenylphosphine, triphenylphosphine oxide, disubstituted azodicarboxylate and reduced hydrazine-derivatives thereof.